Heretofore, it has been known that electron emission could be obtained from a porous billet, as for example a porous tungsten billet that had been impregnated with a barium containing compound such as Ba.sub.3 Al.sub.2 O.sub.6. The Ba.sub.3 Al.sub.2 O.sub.6 impregnate reacts with the wall of the porous tungsten billet generating free barium. The free barium then migrates to the surface by Knudsen flow to give electron emission.
Specific examples of cathodes formed by this general model are described in several U.S. Patents issued to some of the inventors herein. The following is a list of some of these U.S. Patents, all of which are incorporated herein by reference: U.S. Pat. No. 5,114,742, entitled, "Method of Preparing an Improved Scandate Cathode;" U.S. Pat. No. 5,074,818, entitled, "Improved Scandate Cathode;" U.S. Pat. No. 4,895,699, entitled, "Barium Peroxide, Iridium and Excess Tungsten as Impregnants for Cathodes;" U.S. Pat. No. 4,840,767, entitled, "Method of Making a Cathode from Tungsten and Iridium Powders Using a Barium Iridiate Formed from Barium Peroxide and Iridium Oxide as the Impregnant;" and U.S. Pat. No. 4,818,480, entitled, "Method of Making Oxyanion using BaO.sub.2 with Ir and Os or Rh for Cathode Impregnation." These types of cathodes have several uses in vacuum electronic devices that employ thermionic cathodes. These uses include: microwave/millimeter wave tubes, high-speed/high current switches, linear accelerators, X-ray generators for medical and industrial diagnostics, and high power terahertz (infrared) or rf sources.
Other types of cathodes, such as oxide coated cathodes used primarily in televisions, have been disclosed in other U.S. Patents, such as U.S. Pat. No. 4,855,637 issued to Watanabe et al. on May 8, 1990 and entitled, "Cathode for Electron Tube," and U.S. Pat. No. 5,118,984 issued to Saito et al. on Jun. 2, 1992 and entitled, "Electron Cathode Tube." Although these U.S. Patents describe specific embodiments of cathodes, in the Background of these patents, it is generally disclosed that as a result of the reactions of BaO and Si and BaO and MgO, a part of the alkali earth metal oxides, which are formed on the base, are reduced to be an oxygen deficient semiconductor, which facilitates electron emission. These patents further disclose that such cathodes have a current density of 1.32 to 2.64 A/cm.sup.2 at a temperature of 700.degree. C. to 800.degree. C. Despite these results, it is still desired in the cathode arts to attain an electron emission having a current density which is still higher than that current density disclosed by all the above listed U.S. Patents while being able to be operated at lower temperatures.